Opiates remain among the most useful and important class of drugs in medicine, but not without problems. This submission proposes to examine two clinically relevant issues in opioid use ? safety and tolerance. Most opioids used clinically act through the mu opioid receptor. The mu opioid receptor gene Oprm1 undergoes alternative splicing to generate a family of opioid receptors that can be categorized into three classes based upon their structure, each of which contain a number of variants. Knockout mouse models that selectively remove different sets of Oprm1 variants suggest that morphine acts through only one of these sets of variants while drugs acting through different sets of mu receptors lack respiratory depression, physical dependence and reward while maintaining their analgesic activity, thus enhancing their safety. The focus of this application is to understand the role and significance of the sets of mu opioid receptor splice variants in opioid analgesia, side-effects and tolerance. The current application will explore this concept by generating a mouse model in which selected opioid receptor splice variants can be expressed under native control of the Oprm1 gene, thereby permitting the exploration of their actions in vivo. The second aspect of this application involves tolerance. Preclinicla models reveal that short-term opioid administration leads to progressive tolerance. Yet, cancer patients can be maintained on fixed opioid doses without dose escalation to relieve their pain for many months. In a recent study using an extended chronic administration paradigm we reconciled these observations, showing a progressively increasing tolerance to morphine for up to three weeks that then stabilized with no further increases for as long as 6 weeks. Furthermore, this stabilization was associated with changes of select Oprm1 splice variants in specific brain regions of as much as 400-fold. The second component of this application will explore the stabilization of opioid tolerance with extended administration and potential mechanisms.